FR3042126A1 - INSTALLATION AND METHOD FOR EXHAUST GAS PURIFICATION OF AN ENGINE OF A SEA VESSEL - Google Patents

Abstract

This installation comprises a desulfurization scrubber (100), inside which gases to be purified (1) are sent and which is provided with elements (101) for dispersing a liquid (2) for washing these gases, which are adapted to spread the washing liquid inside the scrubber; a tank (102) for collecting effluents (4) from the scrubber; and means for supplying the dispersion elements with washing liquid, these supply means comprising a pipe (200) for circulation of the washing liquid, the outlet of which is connected to the dispersion elements, the inlet of which is connected to a water inlet (7) and / or the collection tank, and which is provided with a pump (201) for driving the washing liquid to the dispersion elements. According to the invention, the feeding means further comprise a system (300) for adding magnesia milk (9) to the washing liquid (2), which includes a source of milk of magnesia (301) and which is adapted to introduce the milk of magnesia from this source into the pipe at an introduction point (303) located between the inlet and the outlet of the pipe, the pipe being designed so that the passage time of the milk of magnesia since the point of introduction to the dispersing elements is between 0.5 and 20 seconds.

Description

Installation and method for cleaning exhaust gases from an engine of a marine vessel

The present invention relates to an installation and a method for cleaning exhaust gases from an engine of a marine vessel.

Marine vessels, whether ships or other vessels, primarily use fuel oil as fuel for diesel engines propelling them. This fuel contains up to 5% by weight of sulfur, most often between 0.5 and 3.5% by weight. During the combustion process in the engines, this sulfur is converted into sulfur dioxide (SO2). As a result, the exhaust gases from these engines are acidic and must be purified before being released to the atmosphere.

The most commonly proposed technology is to use seawater to wash the gases to be purified, most often in open loop: because of its alkalinity, seawater captures sulfur dioxide in a scrubber. inside which, at the same time, the gases to be purified are sent and the sea water is diffused.

In certain situations, for example when the ship is in a harbor or when the natural alkalinity of the seawater is not sufficient, which is typically the case when the ship sails or wets in fresh water an additional alkalinity must be added by a basic reagent. It is conventionally proposed to add sodium hydroxide (NaOH). However, the use of soda poses security problems. In addition, the extreme pH of the sodium hydroxide solutions used, even when diluted, favors an excessively rapid absorption of the sulfur dioxide, inducing risks of formation of precipitates, such as calcium sulphates, contained in the seawater, and Sodium sulphates, these precipitations encrusting the scrubber and / or plugging the elements of this scrubber intended to spread the washing liquid. In addition, the pH profile generated inside the scrubber, which is too high at the top of the scrubber, drops too fast down the scrubber, which affects the efficiency of the gas scrubbing.

The object of the present invention is to provide an installation and a method of purifying gas by washing with water, which, while remaining economical, are more effective and less restrictive to implement. For this purpose, the subject of the invention is an installation for purifying the exhaust gases of an engine of a marine vessel, which installation comprises: a desulfurization scrubber, inside which gases to be purified are sent and which is provided with dispersing elements of a washing liquid of these gases, which are adapted to spread the washing liquid inside the scrubber, - a collection tank effluent washer, and - means supplying the dispersion elements with washing liquid, these supply means comprising a circulation pipe for the washing liquid, the outlet of which is connected to the dispersion elements, the inlet of which is connected to an inlet of water and or at the collection tank, which is provided with a pump for driving the washing liquid towards the dispersing elements, characterized in that the feeding means furthermore comprise a system for adding milk of magnesium. attached to the washing liquid, which includes a source of milk of magnesia and which is adapted to introduce the milk of magnesia from this source into the pipe at an introduction point located between the inlet and the outlet of the pipe, and in that the pipe is designed so that the passage time of the milk of magnesia from the point of introduction to the dispersion elements is between 0.5 and 20 seconds. The subject of the invention is also a method for purifying the exhaust gases of an engine of a marine vessel, in which: the gases to be purified are sent inside a desulphurization washer; washing liquid of the gases to be purified is spread inside the scrubber by dispersing elements, - the effluents of the scrubber are collected in a collection tank, and - the washing liquid circulates in a pipe, the outlet of which is connected to the dispersion elements for supplying washing liquid and whose inlet is fed with water and / or the collection tank, characterized in that milk of magnesia is added to the washing liquid by being introduced into the pipe at an introduction point located between the inlet and the outlet of the pipe, and in that the washing liquid is entrained in the pipe to the dispersion elements so that the passage time of the milk magnesia from the point of introduction to the dispersing elements is between 0.5 and 20 seconds.

One of the basic ideas of the invention is to use, as a basic reagent to add to the washing liquid spread in the exhaust gas desulphurization washer, to milk of magnesia, in other words a suspension of magnesia in the water. By providing, in accordance with the invention, both this milk of magnesia is added in the washing liquid directly into the circulation pipe of the washing liquid, and therefore downstream of the effluent collection tank of the scrubber and upstream of the scrubber. washing liquid inside the scrubber, and that the time taken by the milk of magnesia to pass from its point of introduction into the pipe to the dispersion elements is small, in this case between 0.5 and 20 seconds, several advantages are ensured: - the basic reagent is better used, without losses due to oxidation in the collection tank or related to the deconcentration purges of the washer made by this tray; the pH induced by magnesia is much lower than that induced by lime or sodium hydroxide, which clearly reduces the risk of precipitation of salts, for example calcium carbonates and sulphates and / or sodium sulphites, both at the level of the dispersing elements of the washing liquid, generally located at the top of the scrubber, and inside the scrubber downstream of these dispersing elements; and the fact that the magnesia dissolves slowly, especially compared to lime or soda, the pH profile in the scrubber falls more slowly, this gradual dissolution of the magnesia ensuring that a pH zone favorable to the uptake of the Sulfur dioxide and oxidation of sulfites is maintained longer, which increases the efficiency of the exhaust gas scrubbing.

Thus, the installation and the method according to the invention are particularly effective, while being easy and economical to implement.

According to additional advantageous features of the installation and the process according to the invention, taken separately according to all the technically possible combinations: the source of milk of magnesia consists of an aqueous suspension of magnesia (Mg (OH) 2) having a concentration of 30 to 60% by weight; - the point of introduction is downstream of the pipeline pump; the point of introduction is upstream of the pump of the pipe; - the collection tank is adapted to admit a flow of air; the milk of magnesia, which is added to the washing liquid, consists of an aqueous suspension of magnesia (Mg (OH) 2), having a concentration of 30 to 60% by weight; - a flow of air is admitted into the collection tank. The invention will be better understood on reading the description which follows, given solely by way of example and with reference to FIG. 1 which is a diagram of an installation according to the invention.

In FIG. 1, a motor, in particular a diesel engine, emits exhaust gases 1 which are sent inside a desulfurization washer 100, that is to say a scrubber designed to catch the sulfur dioxide (SO 2) contained in the exhaust gas 1, washing these gases inside the scrubber with a washing liquid 2. The scrubber 100 is embarked on the aforementioned boat. The purified gases 3, leaving the scrubber 100, are released into the atmosphere, if necessary after having undergone non-limiting downstream treatments of the invention.

The washer 100 is a technology known per se. This is for example a packed washer, a rain spray washer or a tray washer. In all cases, to spread the washing liquid 2 inside the scrubber 100 in order to wash the gases to be purified 1, the scrubber is equipped with elements 101 for dispersing the washing liquid 2. These dispersing elements 101 are generally located at the top of the scrubber 100, for example above the lining when the scrubber includes such a lining. By way of nonlimiting examples, the dispersion elements 101 are spray nozzles, flow ramps and / or spray heads.

The effluents 4 of the scrubber 100, that is to say the waters having captured the sulfur dioxide during their passage inside the scrubber after being spread therein, are collected in a tank 102. According to an optional arrangement, the collection tank 102 is supplied with a flow of air 5 which is admitted, in particular injected, directly into the effluents 4 collected: this air stream 5 makes it possible to promote the oxidation of sulfur, by promoting the passage of sulphites to sulphates. In practice, the collection tank 102 is either integrated in the bottom of the scrubber 100, or remote or separated from the scrubber 100, in the form of an auxiliary tank. In any case, an evacuation 6 of the collection tank 102 ensures the regular discharge of the effluents collected, if necessary after the latter have undergone downstream treatment, this evacuation 6 thus allowing a purge of deconcentration vis-à-vis the scrubber 100.

The washing liquid 2 supplying the dispersion elements 101 consists almost entirely of water, the latter being either seawater or freshwater, taken from the sea, the fjord, the estuary, the basin, etc., in which the boat sails or wets, or effluents of the scrubber 100, recycled from the bottom of the latter. A pipe 200 circulates the washing liquid 2 to the dispersion elements 101. In practice, this pipe 200 comprises one or more conduits for conveying the washing liquid. Whatever the embodiment of the pipe 200, the outlet of the latter is connected to the dispersion elements 101, while its inlet is connected, at the same time, to a fresh water inlet 7, typically water seawater or fresh water surrounding the ship, and the collection tank 102 by recycling 8. In operation, the supply of the pipe 200 depends on the operating configuration of the installation: - when the installation works in open loop, the recycling 8 has a zero flow, so that the liquid entering the pipe 200 is entirely made of fresh water from the water inlet 7; when the installation operates in a closed loop, the liquid entering the pipe 200 is almost entirely constituted by effluents from the scrubber 100, coming directly from the collecting tank 102 via the recycling 8, the flow of the fresh water inlet 7 being marginal by serving only to supplement the deconcentration purges discharged by the evacuation 6, as well as to compensate for a possible net evaporation.

Of course, the installation can operate in a semi-closed loop, in the sense that the liquid entering the pipe 200 has parts, respectively for the flow from the inlet of fresh water 7 and the flow of recycling 8 , which are substantial. In practice, valves, not shown in Figure 1, to control these two flows according to the desired operating configuration.

In all cases, the liquid entering the pipe 200 is driven to the outlet thereof by a pump 201 which is equipped with the pipe. This pump 201 is a technology known per se, its embodiment is not limiting.

In operational situations where the natural alkalinity brought by the arrival of water 7 is not sufficient, whether because the boat is sailing in the fresh water of an estuary or a fjord, or because that the part of the flow coming from this inlet 7 is too low at the inlet of the pipe 200 with respect to the stream of the recycling 8, a basic reagent must be added in the washing liquid 2: this basic reagent provides a complement of necessary alkalinity the neutralization of the acidity provided by the sulfur dioxide to be collected in the scrubber 100. According to an essential characteristic of the invention, this basic reagent consists of a milk of magnesia, that is to say of a suspension of magnesia (Mg (OH) 2) in water. For this purpose, concentrated milk of magnesia, in particular having a concentration of between 30 and 60% by weight, is provided in the form of a source 301 and the milk of magnesia of this source 301 is added to the washing liquid 2 , in the form of a stream 9, in the pipe 200. In practice, a pipe 302, controlled by a valve not shown in Figure 1, directly connects the source 301 to the pipe 200, the pipe 302 and the source 301 belonging to thus to a system 300 for adding milk of magnesia to the washing liquid 2. The point 303 for connecting the pipe 302 to the pipe 200, in other words, more generally, the point of introduction of the milk of magnesia to the The interior of the pipe 200 is located between the inlet and the outlet of this pipe. In the exemplary embodiment considered in FIG. 1, this introduction point 303 is downstream of the pump 201, however, it being noted that, by way of a variant that is not shown, the point of introduction can be provided upstream of the pump 201. pump. In all cases, it is understood that, upstream of the point of introduction 303, the washing liquid flowing in the pipe 200 consists entirely of the stream of the water inlet 7 and / or the stream of the recycling 8, while, downstream of the point of introduction 303, the washing liquid consists of the mixture between the liquid flowing in the pipe upstream of point 303 and the milk of magnesia of stream 9.

Compared with soda and lime, the use of milk of magnesia for the basic reagent stream 9 is advantageous. The table reproduced below was established for an exemplary case in which a flow of 140,000 Nm3 / h of gas to be purified 1 containing 320 kg / h of sulfur dioxide is washed, operating at 35 ° C, with 1000 nf / h of sprayed seawater, having an alkalinity of 1000 pmol / l.

When operating without adding a basic reagent (column A of the table), the purification efficiency of gases 1 is only about 80%, which is insufficient to meet the requirements generally imposed. Also, to meet a requirement of 97% purification efficiency, it is carried out by adding sodium hydroxide to the washing liquid sent to the dispersion elements 101 (column B of the table): the pH of the mixture between the seawater and the soda, spread inside the scrubber 100, then rises to about 10.4 at the top of the scrubber, that is to say at the outlet of the dispersing elements. Although the pH then drops rapidly downward into the scrubber, such a high pH value at the dispersing elements 101 induces the risk of precipitation of salts, such as sodium sulfites and calcium sulfates, which tend to blocking the dispersing elements 101 and encroaching the rest of the scrubber 100.

Thus, the case of column A leads to an unacceptable purification efficiency while the case of column B satisfies the purification efficiency but may affect the efficiency of the washer 100 due to its encrusting. It will be noted that the use of lime in place of sodium hydroxide results in a case similar to that of sodium hydroxide with regard to the pH and creates even more precipitation of salts at the level of the dispersion elements 101.

On the other hand, by operating with milk of magnesia for the basic reagent of stream 9 (column C of the table above), the purification efficiency remains satisfied as in the case of lime and soda, without altering the efficiency of the washing of the gases 1, or even increasing the efficiency of the scrubber. Indeed, the pH of the washing liquid obtained at the level of the dispersion elements 101 is of the order of 9, which reduces the risk of salt precipitation. In particular, compared to sodium sulfites and hydrogen sulfites, magnesium sulfites and hydrogen sulfites are much more soluble. Another advantage provided by the milk of magnesia is that the magnesia is not very soluble in water and its dissolution is slow, compared to soda and lime: the pH profile in the scrubber 100 falls less rapidly and an alkaline zone exists longer inside the scrubber. In other words, the alkalinity brought by the milk of magnesia is released more slowly, during the downward flow of the washing liquid 2 spread in the scrubber, from the dispersing elements 101. Insofar as such alkaline zone is more favorable to the capture of sulfur dioxide than an acid zone, such an acid zone settling very quickly with soda or lime, the efficiency of the scrubber 100 is increased, all things being equal. elsewhere. On the other hand, the necessary oxidation of sulphites to sulphates is improved by the persistence of this alkaline zone because the yield of the oxidation reaction is strongly dependent on the pH and decreases rapidly with it.

Insofar as the kinetics of dissolution of magnesia plays an important role for the implementation of the invention, an essential characteristic of the latter is that the time taken by the milk of magnesia to go from its point of addition to washing liquid 2, until dispersed in the scrubber, must remain low. Thus, the passage time of the milk of magnesia from the point of introduction 303 to the dispersion elements 101 is between 0.5 and 20 seconds. In practice, this passage time can be defined as the length of piping existing between the point of introduction 303 and the dispersing elements 101, divided by the speed of the washing liquid 2 in the pipe 200 downstream of the point 303: to For example, when the washing liquid 2 circulates in the pipe 200 at a speed of 2 m / s, the length of the pipe 200 between the point 303 and the elements 101 is approximately between 1 and 40 m.

It is understood that by being introduced into the washing liquid 2 upstream of the dispersing elements 101, the milk of magnesia of the added flux 9 mixes efficiently with the rest of the washing liquid, the suspension of magnesia being distributed homogeneously in the washing liquid downstream of the point of introduction 303.

Similarly, introducing the milk of magnesia in the pipe 200, and therefore downstream of the collection tank 102, has a real technical interest, while going against the usual practices of the field. Indeed, by introducing the basic reagent in the pipe 200 and not in the collection base 102, it avoids the dilution of this reagent in a large volume, and a loss of a fraction of this reagent by the evacuation 6 On the other hand, when the plant operates in a closed or semi-closed loop, the soda or lime is usually introduced directly into the collection tank 102, dosing this intake by measuring the pH in the tank or by enslaving the flow of this basic reagent to a residual sulfur dioxide value in the purified gases 3. However, the effluents 4 collected in the tank 102 contain a mixture partially consisting of sulphites, hydrogen sulphites and sulphates, the oxidation of which is advantageously provided in the tray 102: by introducing the alkaline reagent directly into the collection tank 102, a portion of this reagent serves to correct the excess acidity that the skilled person knows to be generated pa this oxidation, the sulfuric acid produced by oxidation being stronger than the sulfurous acid.

Thus, the invention cleverly combines the use of milk of magnesia and the introduction of this basic reagent both downstream of the collection tank 102 and upstream of the dispersing elements 101, with a limited passage time to reach these levels. scattering elements. Conversely, the introduction of magnesia directly into the collection tank 102 would ultimately result in excessive use of the reagent, because of the losses due to the oxidation in this tank and to the evacuation, especially to the purges, of the contents of this tray, while the use of another reagent such as lime or soda would lead to a pH profile too high at the top of the washer 100 and falling too quickly down the scrubber. In other words, by adding the milk of magnesia in the washing liquid 2 via the point of introduction 303 of the pipe 200, and not by the collecting tank 102 which typically has a residence time of the order of one minute the invention takes advantage of the fact that the dissolution of magnesia occurs inside the scrubber, that is to say where it is most needed, by creating a favorable pH profile.

Claims (8)

1. - Installation for cleaning the exhaust gases of an engine of a marine vessel, which installation comprises: - a desulfurization scrubber (100), inside which gases to be purified (1) are sent and which is provided with elements (101) for dispersing a liquid (2) for washing these gases, which are adapted to spread the washing liquid inside the scrubber, - a tank (102) for collecting the effluents (4) the washer (100), and - means (200, 300) for supplying the dispersing elements (101) with washing liquid (2), these supply means comprising a pipe (200) for circulating the washing liquid, the outlet of which is connected to the dispersion elements, the inlet of which is connected to a water inlet (7) and / or to the collection tank (102), and which is provided with a pump (201 ) for driving the washing liquid to the dispersion elements, characterized in that the supply means (200, 300) comprise in addition, a system (300) for adding magnesia milk (9) to the washing liquid (2), which includes a source of milk of magnesia (301) and which is adapted to introduce the milk of magnesia from this source. source in the pipeline (200) at an introduction point (303) located between the inlet and the outlet of the pipe, and in that the pipe (200) is designed so that the passage time of the milk of magnesia ( 9) from the point of introduction (303) to the dispersing elements (101) is between 0.5 and 20 seconds. 2. - Installation according to claim 1, characterized in that the source of milk of magnesia (301) consists of an aqueous suspension of magnesia (Mg (OH) 2), having a concentration of 30 to 60% by weight. 3. - Installation according to one of claims 1 or 2, characterized in that the point of introduction (303) is downstream of the pump (201) of the pipe (200). 4. - Installation according to one of claims 1 or 2, characterized in that the point of introduction (303) is upstream of the pump of the pipe. 5. - Installation according to any one of the preceding claims, characterized in that the collection tank (102) is adapted to admit an air flow (5). 6. - Process for purifying the exhaust gas from an engine of a marine vessel, wherein: - gases to be purified (1) are sent inside a desulfurization scrubber (100), - a liquid (2) for washing the gases to be purified (1) is spread inside the scrubber by dispersing elements (101), - the effluents (4) of the scrubber (100) are collected in a collection tank ( 102), and - the washing liquid (2) circulates in a pipe (200), the outlet of which is connected to the dispersion elements (101) to supply them with washing liquid and whose inlet is fed with water. water (7) and / or the collection tank (102), characterized in that milk of magnesia (9) is added to the washing liquid (2) by being introduced into the pipe (200) at a point of introduction (303) located between the inlet and the outlet of the pipe, and in that the washing liquid (2) is entrained in the pipe (200) to the pipe dispersing agents (101) so that the passage time of the milk of magnesia (9) from the point of introduction (303) to the dispersion elements is between 0.5 and 20 seconds. 7. - Process according to claim 6, wherein the milk of magnesia (9), which is added to the washing liquid (2), consists of an aqueous suspension of magnesia (Mg (OH) 2), having a concentration from 30 to 60% by weight. 8. - Method according to one of claims 6 or 7, wherein an air flow (5) is admitted into the collection tray (102).